Coxsackievirus B infection was proved by virus isolation or rise in virus titer in 26 children during 1972 at the Montreal Children's Hospital. Sixty-nine percent of these infections occurred during July and August. The spectrum of illness associated with the five implicated coxsackievirus B serotypes included gastroenteritis, pleurodynia, pharyngitis, meningitis, and pericarditis. Lower-respiratory tract infection, transverse myelitis, and an infectious mononucleosis-like syndrome were also seen, and coxsackievirus B1 was isolated from a lymphosarcoma. This experience emphasizes the need for appropriate viral studies in determining the cause of childhood illnesses suspected of being infectious in origin.